As the IT technology is gradually developing, the battery capacity and life of a lithium ion secondary battery are also developing together, but the development may be a kind of development in cell design based on LCO which is an existing material.
However, high capacity batteries which have been developed based on a cell design also show limitation in capacity to be used in recent smart devices and electric vehicles and the like. Therefore, there is a need for a new lithium secondary battery material. The capacity of a secondary battery significantly depends on a cathode active material, and therefore, recently, studies have been conducted on lithium metal composite compounds containing Li2MnO3 having a layered structure containing lithium in excess.
High capacity NCM-based cathode active materials having an Li2MnO3—LiMO2 (M is Ni, Co, or Mn) mixed structure, which have been recently highlighted, may exhibit higher capacity than existing cathode active materials, and may be used under high voltage, and therefore, it is possible to prepare a lithium ion secondary battery having a high energy density when the material is used.
However, cathode active materials having a layered structure including Li2MnO3 are structurally unstable after lithium deintercalation, and thus show deterioration in a lifespan characteristic, and have a problem in that it is difficult for the materials to be adopted in electric vehicles and the like due to deterioration in high rate capability caused by an increase in resistance (impedance) during charge and discharge.
In compounds represented by Formula Li1+αNixCoyMnzO2, compounds with α≦0.1 are materials in which the amount of Li2MnO3 is less than 20%, and are materials in which Li2MnO3 is present in a very small amount, or correspond to existing NCM-based materials. In this case, a capacity in a high voltage region is 200 mAh/s or more and a rate capability is 85% or more, which are very high. However, lifespan characteristic at high voltage is very low.
Meanwhile, materials with 0.1<α≦1, which are an object in the present patent, are materials in which the amount of Li2MnO3 is 20% or more, and Li2MnO3 is a material which participates in charge and discharge at 4.5 V or more and has been highlighted as a cathode material for an electric vehicle and the like by increasing capacity and lifespan characteristics at high voltage. However, there is a problem in that rate capability deteriorates due to low electrical conductivity of Li2MnO3.
Until now, normal cathode active materials (including 0.1<α≦1) have been sintered in a temperature range from 800° C. to 950° C. in order to simultaneously secure capacity and lifespan characteristics.
When firing is performed at 800° C. or less, for example, at a low temperature such as 700° C., the capacity is increased to 240 mAh/g or more, but a lifespan characteristic and a rate capability are significantly reduced by approximately 70% due to reduced crystallinity. Accordingly, it is very difficult at the current technical level to prepare a cathode active material which is capable of exhibiting high capacity, and simultaneously shows a lifespan characteristic of 90% or more and a rate capability of 80% or more.
Meanwhile, Patent Document 1 proposes a cathode active material including a lithium manganese oxide represented by Formula Li2MnO3−xAx (here, A is an element having an oxidation number of −1 and a halogen atom such as fluorine and chlorine, or a transition metal element, and 0<x<1) in an amount of 50% or more based on the total weight of the cathode active material, by partially substituting an oxygen element in Li2MnO3 which is inexpensive and excellent in structural stability with an element with a −1 valence.
However, Patent Document 1 only discloses “since a lithium manganese oxide of Formula 1 according to the present invention may be prepared by, for example, a method including mixing ‘a lithium compound’ as a lithium supply source, ‘a manganese compound’ as a manganese supply source, and ‘a metal compound containing A’ as a doping element supply source in a predetermined content range and subjecting the mixture to heat treatment, and the lithium compound, the manganese compound, the metal compound containing A and the like are known in the art, the description thereof will be omitted in the present specification.”, but does not disclose a method for preparing such a cathode active material, basic characteristic conditions of a cathode active material prepared by the method at all.